This invention relates to a method of transmitting clock signals in a synchronous signal communications network to a method of synchronizing clock generators or network elements of a synchronous digital communications network to a network element of a synchronous digital communications network and to a clock generator for a synchronous digital communications network.
In an article by Th. Klett, xe2x80x9cAspekte der Netzsynchronisationxe2x80x9d, Alcatel Telecom Rundschau, 1st Quarter 1997, it is described that in a communications network based on the synchronous digital hierarchy (SDH) standard, the network elements have to be synchronized. According to the article, network nodes include dedicated clock generators which distribute a clock signal to all network elements of the node. The distribution is done either through clock interfaces, so-called 2-MHz interfaces, or through interfaces for 2-Mb links.
For synchronization, synchronization signals (clock signals) are transmitted between the network elements and between the network elements and the clock generator. Either 2-MHz signals are transmitted over dedicated synchronization links (clock interfaces) or use is made of message signals at a bit rate of multiples of 155 MHz, from which a clock signal is derived. Each network element receives synchronization signals from several neighboring network elements, and selects one of the synchronization signals as a reference clock signal based on a synchronization status message (SSM), which indicates the clock quality. In the case of 2-MHz links, however, it is not possible to transmit a synchronization status message. Therefore, 2-Mb links are preferred to the 2-MHz links for the transmission of the synchronization signals. This, however, requires much more complex and expensive interface devices.
It is an object of the invention to provide an alternative method of transmitting synchronization signals wherein quality information can be transmitted with the clock signal and which manages with simpler interface devices. Another object of the invention is to provide a method of synchronizing clock generators or network elements of a synchronous digital communications network wherein clock signals can be received through simple clock interfaces. Further objects of the invention are to provide a network element and a clock generator comprising a clock interface for receiving or transmitting clock signals which makes it possible to transmit quality information with the clock signal.
These objects are attained by a method of transmitting a clock signal serving to synchronize a network element of a synchronous digital communications network, the clock signal being received from a clock source via a clock interface of the network element, the method comprising the step of applying a phase modulation characteristic of the quality of the clock signal to the clock signal.
The invention is further directed to a method of synchronizing a clock generator or a network element wherein a clock signal is received via a clock interface, the method comprising the step of applying a phase modulation characteristic of the quality of the clock signal to the clock signal, detecting the phase modulation and deriving quality information therefrom, and based on the quality information, synchronizing the clock generator or the network element to the clock signal. According to a further aspect of the invention, two or more further clock signals are received which also have a phase modulation characteristic of their quality applied to them, and based on the quality of the two or more further clock signals, one of the clock signals is selected for synchronization.
The invention is further directed to a network element for a synchronous digital communications network, comprising a clock supply circuit for supplying the network element with an internal clock signal, a clock interface for receiving an external clock signal from a clock source, and means for synchronizing the clock supply circuit to the received external clock signal, the network element further comprising means for detecting a phase modulation applied to the external clock signal, said phase modulation being indicative of the quality of the external clock signal. According to a further aspect of the invention, the network element further comprises means for applying a phase modulation indicative of the quality of the internal clock signal to clock signals to be transmitted.
Finally, the invention is directed to a clock generator for distributing a clock signal to network elements of a synchronous digital communications network, comprising clock interfaces for connecting the network elements, and further comprising means for determining the clock quality and means for applying to the clock signal a phase modulation indicative of the quality of the clock signal.
Further aspects and advantages of the invention will be apparent from the detailed description and appended claims below.
Advantages of the invention are that it can be implemented by few, simple technical means, that the quality information is available together with the clock signal throughout the communications network, and that the method according to the invention can be used in both directions, from the clock generator to the network element and from the network element to the clock generator.